Faculty of Technology and Science
Chemical Engineering
Christer Korin
Fracture Behaviour of
adhesive Joints in
carton board
Karlstad University Studies
2007:35
Christer Korin
Fracture Behaviour of
adhesive Joints in
carton board
Karlstad University Studies
2007:35
Christer Korin. Fracture Behaviour of adhesive Joints in carton board
Licentiate thesis
Karlstad University Studies 2007:35
ISSN 1403-8099
ISBN 978-91-7063-138-7
© The author
Distribution:
Karlstad University
Faculty of Technology and Science
Chemical Engineering
SE-651 88 Karlstad
SWEDEN
Phone +46 54 700 10 00
www.kau.se
Printed at: Universitetstryckeriet, Karlstad 2007
Licentiate Thesis, October 2007
Fracture Behaviour of Adhesive Joints in Carton Board
Christer Korin
1
Abstract
A carton-board package is often sealed and closed with an adhesive. Package requirements
vary depending on how the package is to be used. A package that is only supposed to protect
the product during transport differs from one that is supposed to attract consumers and
facilitate the use of the product by consumers. Fracture of the adhesive joint may occur in
several different ways, e.g. cohesive fracture in the adhesive, interfacial fracture between the
adhesive and one of the carton-board surfaces, and cohesive fracture in the carton board. The
traditional way of testing the adhesive joint is to subjectively evaluate the fibre tear after
manually tearing the joint apart. The adhesives used in carton-board packages are either hotmelt adhesives (adhesives that are applied in a molten state on the carton board) or dispersion
adhesives (adhesives that are applied as water-based dispersions).
The primary interest of this study has been to find an objective method that can characterise
the adhesive joint, i.e. the strength and joint characteristics. The work has principally
concentrated on physical experiments where the Y-peel method is evaluated and further
developed, including construction of a laboratory adhesive applicator.
Adhesive joint failure is analysed and correlated to the force-elongation curve in order to
explore various mechanisms of adhesive-joint failure. The force versus elongation curves are
transformed into a force versus inelastic deformation curve for the adhesive joint. The
inelastic deformation of the adhesive joint is defined as the inelastic opening of the adhesive
joint perpendicular to the carton-board surface. The dissipative descending energy has been
used to evaluate the adhesive joint. High descending dissipative energy resulted in high
resistance against final failure of the joint. This correlates very well with the manual fibre-tear
test. Characteristic force-elongation curves in Y-peel testing, i.e. the shape of the curve, have
been analysed and found to have four main failure modes.
Using both the newly designed adhesive applicator and the Y-peel test method revealed a
large discrepancy, about three orders of magnitude, between the theoretical thermodynamic
analysis and the experimental test result. The new objective method can be used to design the
interaction between the adhesive and the carton-board surface for a specific application. This
can be achieved by modifying the carton board, the adhesive or the process parameters.
2
Papers included in the thesis
J. Tryding, M. Lestelius, C. Korin and J. Lewandowski, Interpretation of Y-peel testing of
adhesive sealed carton board, in: Verarbeitungsmaschinen und Verpackungstechnik Dresden
2003 pp. 277-294 (2003). (Paper 1)
C. Korin, J. Tryding. M. Lestelius, N. Hallbäck, Y-peel characterization of adhesively-bonded
carton board-An objective method, Journal of Adhesion Science and Technology 21 (2): 197210 (2007) (Paper 2)
C. Korin, N. Hallbäck and R. Junghans, Failure modes of adhesive joints in carton board,
Submitted for publication (2007) (Paper 3)
3
Table of contents
Table of contents ......................................................................................................................
1. Introduction ........................................................................................................................ 5
1.1 Carton board ................................................................................................................. 6
1.1.1 Transport and consumer packages ........................................................................ 7
1.2 Sealing .......................................................................................................................... 7
1.2.1 Slid sealing ............................................................................................................ 8
1.2.2 Closure sealing ...................................................................................................... 8
1.2.3 Adhesives .............................................................................................................. 8
1.2.4 Adhesive-application process................................................................................ 8
1.3 Demands on the joint.................................................................................................... 9
1.3.1 Problem ............................................................................................................... 10
1.3.2 Research .............................................................................................................. 10
2. Method ............................................................................................................................. 10
2.1 Adhesive applicator.................................................................................................... 11
2.2 Y-peel ......................................................................................................................... 12
2.2.1 Force elongation .................................................................................................. 13
2.2.2 Interpretation and failure characterisation........................................................... 14
3. Theories ............................................................................................................................ 15
3.1 Fracture, wetting and adhesion................................................................................... 15
4. Results and discussion...................................................................................................... 18
4.1 Typical failure modes................................................................................................. 19
4.2 Characterisation of failure modes............................................................................... 20
4.3 Y-peel characterisation and fibre tear ........................................................................ 21
5. Discussion ........................................................................................................................ 22
5.1 Y-peel test method ..................................................................................................... 22
5.2 Y-peel characterisation............................................................................................... 22
5.3 Y-peel failure.............................................................................................................. 23
5.4 Demands on a joint..................................................................................................... 24
6. Conclusions ...................................................................................................................... 24
7. Acknowledgements .......................................................................................................... 25
8. Reference.......................................................................................................................... 26
9. Notations .......................................................................................................................... 28
4
1. Introduction
People have always made packages and receptacles in which to protect their goods during
storage and transport. When people first started collecting and transporting materials,
packages were made of natural materials suited to the contents of the package [1]. Examples
of packaging from this period are earthenware jars, wooden cases, leather cases or even
leaves. As technology and marketing concepts developed, the demand for more complex and
sophisticated packages to meet transport demands, storage conditions and display provisions
emerged on the market. At the dawn of the industrial era, mechanical converting machines
expanded the possibilities for optimising package functionalities and creating new designs of
modern packages. Today almost every product is packed in some sort of package during its
entire product life or part of it. The products that are packed today are very diversified, e.g.
ball bearings, batteries, toys, food, liquids, medicine and luxury products; e.g. cosmetics,
liqour each of these products has its own requirements on the package. The package material
is also much more varied, e.g. paper, glass, metal and plastic.
The traditional role of the package was to protect the product for the consumer during storage
and transport. An additional important and increasing demand today from consumers and the
industry is to make the package an integrated part of marketing. Thus, the initial function of
the package evolved from not only protecting the product during transport and storage but
also to informing and displaying data on it, as well as providing the consumer with a positive
overall experience of the contained product [2], from when the product is sold until when it is
used and disposed of. When the consumer decides to pick and buy a product off the shelf, the
package has a large influence as a “silent salesman” [3]. The package/product on the shelf that
catches attention and looks good is chosen. This is called “the first moment of truth”. “The
second moment of truth” [2] is when the consumer decides to buy the product a second time,
and this depends on the relation between the consumers’ expectations and experiences when
using or consuming the product. The combination of the package and its product thus creates
the total experience of the product.
The process of producing a packaged product from carton-board reels includes several
converting steps such as printing, embossing, creasing, cutting, punching, folding, gluing,
erecting, filling and sealing. This can be done in one single in-line converting process or in
several discrete stand-alone machines that cover one or more converting steps. Each
converting step is a scientific and technical challenge in itself.
Packages can be sealed several different ways, such as by gluing, stapling or mechanical
interlocking. This thesis will focus on the process of gluing, material properties, and process
conditions affecting the performance of the adhesive joint. The mechanical strength of
adhesive joints in carton-board packages is an important product property of the package so
that the sealed package meets the demands on it throughout its complete life cycle. A reduced
operating window caused by increasing production speed and automation leads to an
increased demand for an objective method for testing the mechanical strength of adhesive
joints. How to optimise package sealing for a specific package application becomes more and
more important because a failure in the converting process causes more losses in productivity
when increasing speed in the production process. Using the wrong adhesive can turn out be
too expensive with respect to either customer problems or reduced profit for the producer. The
joint has to fulfil expectations throughout the complete life cycle of the package from
converting, distribution, consumer and recycling perspectives. It is important to have an
objective and quantitative joint testing method as a tool for gaining more knowledge about the
5
quality of adhesive joints in carton-board packages and for optimising the cost-performance
relations in the entire value chain.
1.1 Carton board
Carton board is usually defined as a paper material ranging from about 200 g/m2 up to 600
g/m2. If the grammage is less than 200 g/m2, the material is usually called paper; if it is higher
than 600 g/m2, millboard. Carton board is often made as multiply board (sandwich
construction). With this type of multiply board, the producer has the possibility to design the
properties of each ply to meet various demands for different applications. Carton board is an
anisotropic material with different properties in the in-plane directions – the machine
direction (MD), which is the direction in which the board is produced on the board machine,
and the cross-machine direction (CD) – and the out-of-plane direction – the thickness
direction (ZD). The in-plane stiffness properties are 100 times or more higher than the out-ofplane stiffness; see Table 1 [4]. Furthermore, the in-plane tensile yield stress is at least ten
times higher than the failure stress in the out-of-plane direction.
Table 1. Experimental results of uniaxial tensile tests in the MD and CD and of throughthickness tensile tests in the ZD of a multiply board [5].
Elastic
modulus [MPa]
MD 5600
CD 2000
ZD 18
Tensile yield
stress [MPa]
12
6.5
-
Tensile failure
stress [MPa]
44
18
0.40
The carton board used in this thesis is a four-ply carton board from Korsnäs Frövi. The carton
board is built up from the bottom ply with unbleached softwood sulphate pulp, and in the two
middle plies, there is a mixture of unbleached softwood sulphate pulp, unbleached softwood
chemi-thermo-mechanical pulp (CTMP), and internally produced broke from the board
machine. The top ply is a mixture of softwood and hardwood bleached sulphate pulp, and on
top of that, there is a pigment-coated surface.
FRÖVI LIGHT
Coating
Bleached Cellulose Pulp
Unbleached Cellulose Pulp and
CTMP
Unbleached Cellulose Pulp and
CTMP
Unbleached Cellulose Pulp
Figure 1. The Korsnäs Frövi carton board LIGHT 330 in cross section, both schematically and
as a SEM micrograph.
6
1.1.1 Transport and consumer packages
Package requirements vary depending on how the package is to be used. A package that is
only supposed to protect the product during transport differs from one that is supposed to
attract customers and facilitate the use of the product by consumers.
The board and the transport package must withstand the stresses – such as folding, stacking,
water/moisture and vibration – that it will be exposed to during converting, filling/loading and
transportation. Smaller consumer packages can be packed in a transport package for
distribution from the manufacturer to the supermarket or shop. The size of the transport
package is then adapted to contain the number of consumer packages that is convenient for
the shop or supermarket to handle. The different sizes and applications define the specific
demands on the sealing until the package is opened. In addition, the transport package may
also be used as a display package, i.e. a package that urges the consumer to buy the product
from the shelf in the shop, which adds further demands on the sealing. In this case, it is
important that the sealing opens as desired.
The consumer package must withstand the stresses that the material and the package are
exposed to during the complete lifecycle of the package, from converting the blanks to
erecting, filling, transportation, shelf exposure, consumption of the content to the final
recycling of the package and/or the material. Hence it follows that the production and
transportation demands placed on the adhesive joint, the package must also live up to the
expectations of the consumer to reinforce the total product experience [2]. From the
consumer’s point of view, the transport package will be useless immediately after opening
(e.g. a transport box for a refrigerator), whereas the consumer package may be used for a
longer period of time (e.g. packages for breakfast cereals, butter, fruit juices or cosmetics).
The demands on adhesive joints differ depending on whether the package is a transport
package, consumer package or combined transport/consumer package. It is not desirable that
first-time opening destroys the appearance or function of the package if the package is
elaborately decorated and if it will be used by consumers over a long period or if it will be
used as a display package on a store shelf. In such cases, fracture involving tearing or
delaminating is not desired. On the other hand, maximum toughness may be preferred
irrespective of the fracture process for packages where the entire content will be used at once.
In some cases, it is desirable that the adhesive joint opens with a distinct snap-like manner.
This requires low toughness but high strength in order for the package to resist converting and
withstand transportation to the consumer.
1.2 Sealing
Fracture of the adhesive joint may occur in several different ways, e.g. cohesive fracture in
the adhesive, interfacial fracture between the adhesive and one of the carton-board surfaces,
and cohesive fracture in the carton board. Most of this bonding is due to chemical interactions
across the interface such as Lifshitz-van der Waal and acid-base interactions (cf. [6]). In
addition, diffusion and entanglement of polymer molecules between the substrate and the
adhesive may increase the adhesion; mechanical interlocking between the adhesive and the
substrate may also have the same effect.
7
1.2.1 Slid sealing
Here slid sealing is defined as all kinds of sealing that occur before filling. These sealings are
generally not opened when the content is being used. Therefore it is important that this type of
joint is intact and closed during the complete package chain. The demand on this type of joint
is to resist the stresses and not to fail. If the joint eventually fails, then the character of the
failure is normally of no interest.
1.2.2 Closure sealing
The closure sealing, here defined as the closure of a filled carton-board package, is sometimes
easy to open with a short click sound, and in other cases, it is tamper-proof. When a package
is nicely decorated and is to be used over an extended period of time, then it is desirable that
the first-time opening of the seal is easy and does not damage the appearance of the package
through fibre tear. On the other hand, when a tamper-proof seal is required, then the opening
of the seal must result in significant fibre tear as an obvious sign or evidence that someone has
opened or tried to open the package.
1.2.3 Adhesives
The adhesives used in carton-board packages are either hot-melt adhesives (adhesives that are
applied in a molten state on the carton board) or dispersion adhesives (adhesives that are
applied as water-based dispersions).
A typical traditional hot-melt adhesive has these main chemical components:
-
30–40% polymer
20–30% wax
30–40% resin
The polymer gives the adhesive its cohesive mechanical strength; i.e. it is the glue substance.
The resin is the wetting substance, whereas the wax gives the viscosity and regulates the
adhesion (i.e. the tack) when the adhesive is applied on the carton board. Normally, a small
amount of antioxidants are also added, and they protect the adhesive from ageing [7].
Hot-melt adhesives often have a chemical basis of ethyl vinyl acetate or polyolefin, and are
designed to suit the carton-board packaging industry. No dispersion adhesives have been used
in this study.
1.2.4 Adhesive-application process
In the machine converting process of producing carton-board packages, there are different
ways of sealing, e.g. stapling, adhesive sealing, adhesive tape and mechanical interlocking.
With increasing machine speed, the use of adhesive seals has increased as well. Adhesives
also make it easier to produce nicely designed packages with high-quality printed decorations
and different, untraditional shapes. At the same time, it is important for the industry that the
converting process runs with very few interruptions and that it has a high and constant quality
because of the fierce competition on the market.
8
Compression
Gluing
Prefolding
Carton
blanks
Figure 2. A schematic illustration over a folder-gluer machine redrawn from [8]
It is important to be able to control the application of the adhesive, i.e. application of the
adhesive on the substrate, wetting of the board by the adhesive, consolidation of the adhesive,
and the final joint formation to control the quality of adhesive joints (the adhesive joint is in
this context defined as two sheets of carton board with a hot-melt adhesive in between). This
control is possible by controlling certain process parameters. The most important parameters
to get the full and intended strength of the joint are:
-
open time
pressure time
pressure on applied adhesive string
amount of applied adhesive
temperature of applied adhesive
To withstand certain package conditions, it is important that the adhesive is applied in such a
manner as to avoid indications of fracture.
1.3 Demands on the joint
In the beginning, the demands on a package were that it was able to be closed and that the
content was protected. The appearance of the seal was of practically of no importance. In
some cases, however, it was important that the seal was tamper-proof. Later during
industrialisation and in machine production, runability was added as an important factor. It
was important for profitability that the machines run fast, smoothly and with few stops. The
optimisation of package sealing for specific package applications became more and more
important because a jam in the converting machine would cause a large drop in productivity.
In addition, poor sealing can be very expensive with respect to customer complaints and loss
of image on the market. The joint has to resist the demands throughout the complete life cycle
of the package.
To optimise the adhesive joint, it is not only necessary to know whether the adhesive joint is
good or bad but also how good or bad. The contribution from mechanical bonding can be
particularly important for carton board where the mechanical bonding is achieved through
mechanical interlocking of the adhesive into irregularities and pores of the carton-board
surface and the embedding of fibres sticking out from the surface (see [9], [10]), and creates
fibre tear. In other cases, pop-up behaviour is required where a low degree of fibre tear is
needed. To enable measurements of adhesive joints in an objective manner, it is appropriate to
use a tensile test machine and measure the force-elongation curve.
9
1.3.1 Problem
The demands on the adhesive joints originate from loading and environmental conditions (e.g.
forces from the surroundings such as other packages, dynamical forces from the machines,
and/or climate changes such as temperature and relative humidity) during converting,
transport, storage, exposure on shelf, end use of the product and finally recycling. Fracture of
the adhesive joint may occur in mainly three different ways:
-
cohesive fracture in the adhesive
interfacial fracture between the adhesive and the carton board
cohesive fracture in the carton board
Fibre tear
No fibre tear
Figure 3. Two Y-peel specimens torn apart, with and without fibre tear.
The failure may also be a combination of the above-listed failure modes. Several different test
methods exist [11] such as the T-peel, angle-peel and Y-peel methods.
The first step is to find a procedure to make adhesive joints in a repeatable way in order to
investigate how the adhesive joint behaves when different specified parameters are changed.
Enhanced wetting will lead to higher work of adhesion (see [12]). The traditional way to test a
joint is to conduct a manual peel test [4] and subjectively evaluate the degree of fibre tear. To
control the converting process, an objective method has been developed; this method includes
a laboratory adhesive applicator and a test procedure that tears apart the adhesive joint in a
well-defined manner.
1.3.2 Research
The purpose with this work is to find an objective test method to characterise how good or
bad an adhesive joint is when exposed to mechanical loading. In addition, the test method
should be repeatable and correlate with practical experience in the converting industry. This
study of adhesive joints is mainly done through physical experiments where the Y-peel
method is evaluated and further developed for the characterisation of the mechanical strength
of adhesive joints in carton-board packages. The adhesive-joint failure is also analysed and
correlated to the force-elongation curve to explore various mechanisms of adhesive-joint
failure. The force-elongation curves are analysed with the expectation to gain knowledge
about the behaviour of the adhesive joint during crack propagation and to make it possible to
predict the behaviour of the adhesive joint during stress.
2. Method
The traditional way of analysing the mechanical strength of a hot-melt adhesive joint in the
converting industry is to look at the fracture surfaces after manual peeling. The joint is
10
acceptable if there are more than 50% fibres on the fractured adhesive surface; otherwise, it is
considered bad. However, this method is very subjective and based on personal skills and
experience. There are other semi-manual peeling methods in which the joint is subjected to a
load, either an increasing load until the joint fails or a constant load, and the time to failure is
measured [13]. Other methods are based on using tensile testing equipment in which the
force-elongation curve is recorded during peeling until the joint fails.
F
F
F
Leg
Base leg
Leg a
Tail
Leg
B
Support block
Stabilizing
Tab
Leg b
Crease
Line
Crease
line
F
F
R
R
Fixed platform
Moving trolley
Figure 4. T-peel, angle-peel and Y-peel [11].
Peel tests suggested in the literature for carton board-based packaging materials are, e.g., the
T-peel tests [13], Figure 4 (left); the angle-peel tests [14], Figure 4 (middle); and the Y-peel
tests [11], Figure 4 (right). These methods all have their advantages and disadvantages. In the
T-peel test method, the adhesive joint is not fixed relative to the tail and the tail is free to
move during the test. Ways to overcome this movement are to stabilise the tail by holding the
tail in a fixed position or by adding a stabilising tab [13], [15].
In the angle-peel and the Y-peel test methods, no arbitrary movement is allowed. It is further
observed that in the angle-peel test, the sample needs to be bonded to a sliding support block.
This makes the preparation work for the angle-peel test more time consuming compared to the
other two methods. The Y-peel method is a redesign of the constrained T-peel test. The Ypeel method is discussed in detail below.
2.1 Adhesive applicator
In this study, a laboratory adhesive applicator was designed in co-operation with IM-Teknik.
A prototype was built, Figure 5, and today fully developed equipment is commercially
available from IM-Teknik, Gothenburg, Sweden. The process by which the adhesive glues
two board surfaces together involves a number of steps [4], and the adhesive applicator is
required to be able to simulate the different process parameters in the adhesive line of a
converting and filling machine.
It is possible to set these different process parameters in the developed adhesive applicator:
-
open time – the time from application of the adhesive on one board surface
until the other sheet is pressed against the adhesive string
pressure time – the time during which the adhesive joint is set under
pressure
pressure on the joint – the pressure that is applied on the adhesive joint in
the adhesive applicator
amount of adhesive – the amount of adhesive that is applied on the sheet
11
-
temperature – the temperatures in the adhesive aggregate tank, hose and
nozzle
application speed
Weights
Transport plate
Press plate
Hot-melt adhesive nozzle
Dispersion adhesive nozzle
Figure 5. Photo of the adhesive applicator, with its various components labelled.
2.2 Y-peel
To produce repeatable test results, the Y-peel equipment was redesigned and adapted to the
Y-peel samples. Sample preparation for the Y-peel test method is fully described by Tryding
et al. [4]. The Y-peel fixture is set up into the uniaxial tensile tester (Figure 6). During a test,
the uniaxial tester pulls the upper clamp with a constant speed and continuously records the
resulting forces with corresponding prescribed deformations. As the upper clamp moves, the
Y-peel specimen stretches, which ultimately leads to a fracture in the adhesive joint of the Ypeel specimen. The Y-peel specimen is mounted in hinged upper and lower clamps so that
movement will be free of friction. The free ends cannot move uncontrolled because they are
fixed in the clamps.
Figure 6. Photo of the fixture with the mounted Y-peel specimen.
12
P
P/2
∆
Ft M
Fn
α
R
R
R
Figure 7. The Y-peel set-up together with a sketch of the geometry and boundary conditions.
∆ is the displacement of the point of fracture opening during testing.
The set-up of the Y-peel method is shown in Figure 7 together with sketches of the geometry
and boundary conditions. In the middle of Figure 7, the load P is acting on the upper simply
supported clamp of the set-up. The resulting reaction forces R at the two simply supported
clamps at the lower part of the set-up are of the same magnitude. The forces are all aligned
parallel to the specimen legs. Force equilibrium gives the relation
P − 2 R cos(α 2) = 0
(1)
where α is the peel angle; see the middle of Figure 7. Using the symmetry line in the Y-peel
set-up gives the reaction forces and moment at the adhesion joint zone; see the right of Figure
7. The reaction forces in vertical and horizontal direction are denoted Ft and Fn , respectively.
The moment is denoted as M . In this set-up, ∆ is considered small enough to justify the
approximation of M≈0. Force equilibrium in the vertical and horizontal planes then gives,
using eq. (1), the relation
Ft = 0 and Fn =
P
tan(α / 2)
2
(2)
respectively. Note that the reaction force, Fn , on the right of Figure 7, is perpendicular to the
board surface (for detailed discussion. see Korin et al. [11]).
2.2.1 Force elongation
The force-elongation curves have to be separated into their elastic and dissipative (inelastic
opening) regions before they can be analysed in a relevant and objective way; see Figure 8
[11].
13
Elastic stiffness
F
FMax
WDdesc
WDasc
Welremain
WDinitial
u
(u-ue)
Figure 8. Typical total force-elongation curves from Y-peel tests and identification of the
elastic component and extraction of the dissipative part [11].
Typical force-elongation curves for a test sample are shown in Figure 8. The area for
extracting the remaining elastic energy is marked Welremain ; the dissipative ascending and
descending energy are marked WDasc and WDdesc respectively [11]). The total elongation is
denoted u , whereas the elastic part of u is denoted u e , and the dissipative part of u is
derived as the difference u − u e . The initial energy caused by initial tightening of the
specimen is called WDinitial .
The crack propagation begins and ends in the dissipative part of the force-elongation regime,
and the fracture development depends on how the crack propagates after its initiation.
2.2.2 Interpretation and failure characterisation
It is possible to derive the inelastic behaviour of the adhesive joint by separating the recorded
force-elongation curve into elastic and dissipative parts (Figure 8). The testing method is
described in the publication [4] and the mathematical derivations in [11]. This is based on the
fact that the proportional limiting yield strength in the in-plane direction is at least ten times
higher than the out-of-plane failure stress [ref 5, 16]. Inelastic deformation is hence confined
to out-of-plane deformation of the adhesive joint. Using the equations
Fn =
F
and the inelastic opening δ nie = 2(u − u e )
2
(3)
[11], [17], the force versus elongation curve can be transformed into a force versus inelastic
deformation curve for the adhesive joint (Figure 9). The inelastic deformation of the adhesive
joint ( δ nie ) is defined as the inelastic opening of the adhesive joint in the direction of Fn .
14
Ascending dissipative
part of the
force-elongation curve, WDasc
Descending part
of the dissipative
energy, W Ddesc
Figure 9. Force (F) versus elongation (u) of an adhesive joint with an ascending and a
descending part, with a pre-force of 3 N to get rid of the initial dissipative energy.
3. Theories
The dissipative energy can be computed from the force versus inelastic deformation curve of
the Y-peel test. The dissipative energy is caused by the formation of cracks, which in turn is
due to the formation of microcracks and the subsequent coalescence of these cracks in the
thickness direction of the carton board, or by inelastic deformation and damage of the
adhesive. Hence, the dissipative energy is a measure of the amount of energy that is consumed
during opening of the joint. As can be seen in Figure 9, the dissipative energy can be divided
into an ascending and a descending part.
The force equilibrium means that the carton board and the tensile test machine will behave
completely elastic, while the adhesive joint will fail out-of-plane (Table 1). The combination
of the relative proportions and the magnitudes of the elastic, ascending dissipative work, and
descending dissipative work [11] make it possible to objectively characterise how good or bad
a certain adhesive joint is for a particular package application. The ascending part is the first
part of the curve that ends at the maximum force ( Fn max ), cf. Figure 9. The energy that is
consumed during the ascending part of force versus inelastic deformation is denoted WDasc .
The descending part of the curve starts at the maximum force and ends where final failure
occurs. During the descending part of force versus inelastic deformation, the coalescence of
microcracks and fibres pulled out from the carton board result in a softening behaviour [5].
Part of the softening behaviour may also be due to the deformation and fracture in the
adhesive as such. In some cases, however, a rapid failure occurs at the maximum force so that
the descending dissipative energy is essentially zero. The energy that is consumed during the
descending part of force versus inelastic deformation is denoted WDdesc .
3.1 Fracture, wetting and adhesion
Fracture of the adhesive joint may occur in several different ways, such as:
-
cohesive fracture in the adhesive
interfacial fracture between the adhesive and one of the carton-board
surfaces
cohesive fracture in the carton board
15
The failure may also be a combination of the above-listed failure modes.
The work of cohesion wCa , i.e. the surface energy per unit area associated with cohesive
fracture in the adhesive, is equal to 2γ a , [6], where γ a is the surface tension of the adhesive.
The work of cohesion in the carton board wCs could similarly be expressed as equal to 2γ s ,
where γ s denotes the surface tension of the carton board.
The work of adhesion in the interface between the adhesive and the carton board is expressed
by the Young-Dupré equation according to
(4)
w A = γ a + γ s − γ as
Assuming that the surface tensions of the carton board and the liquid adhesive in vacuum are
approximately the same as the corresponding surface tensions in adhesive vapour (although
adhesive vapour adsorption to some extent influences the surface tension of the substrate
[18]), then the contact angle θ could be derived from Young’s equation as
cos θ =
γ s − γ as
γa
(5)
The contact angle is the angle between the upper surface of the liquid and the interface
between the liquid and the substrate, when a small liquid drop is placed on a horizontal plane
substrate. Hence, the contact angle θ is a measure of wetting of the substrate by the liquid.
Complete wetting results if (γ s − γ as ) ≥ γ a , leaving θ = 0 . This corresponds to the nonequilibrium situation when the liquid spreads infinitely on its substrate. The expression
(γ s − γ as ) corresponds to the empirical critical surface tension γ c for the substrate [19].
Hence the substrate is completely wetted by the adhesive if γ c for the substrate is greater than
the surface tension of the liquid adhesive. By combining eqs. (4) and (5), the work of
adhesion could be expressed as
w A = γ a (1 + cos θ )
(6)
From eq. (6), it is evident that enhanced wetting gives higher work of adhesion, a fact that has
also been experimentally verified (see [12]). From eq. (6), it may seem as if the work of
adhesion is always less than or equal to the work of cohesion of the liquid adhesive. Note,
however, that for complete wetting γ s − γ as ≥ γ a . As pointed out by [6], this implies that
w A > 2γ a = wCa for complete wetting conditions. This suggests that fracture in such cases
would occur as cohesive fracture in the adhesive, rather than interfacial fracture between the
substrate and the adhesive. This may be true for ideally brittle conditions, but in most cases
the bulk of the material (the adhesive and the carton board) experiences energy dissipation not
only due to crack growth, but also due to inelastic deformation. To take such effects into
account, deeper fracture mechanics must be considered.
Consider two solid sheets of carton board joined together with an adhesive. During the
progressive creation of free surfaces (commonly referred to as crack growth), in the solid
16
board, the adhesive or the board-adhesive interface, energy balance for the body requires that
(see [20])
(7)
∆Ω = ∆W + ∆W f + ∆Wk
where ∆Ω denotes the energy put into the system by external forces, ∆W is the change in
strain energy, ∆W f is the total surface energy consumption during crack growth, and ∆Wk is
the kinetic energy of the body during crack growth. The strain energy can be divided into its
elastic and inelastic parts, ∆Wel and ∆Wie . The elastic energy is the part of the total strain
energy that is recovered during unloading of the body. Eq. (7) can now be rewritten as
(8)
∆Ω = ∆Wel + ∆Wie + ∆W f + ∆Wk
The dissipative energy during crack growth is defined as ∆W D = ∆Wie + ∆W f . Note that
inelastic energy dissipation does not result in a change of elastic stiffness of the body as
opposed to the energy dissipation associated with crack growth. In most cases, the strain
energies vary as a function of position in the body, which means that ∆Wel and ∆Wie are the
integrated elastic and inelastic strain energy densities over the entire volume of the body. For
non-homogeneous conditions, crack growth may occur simultaneously at different locations
in a body meaning that
(9)
∆W f = wCa ∆Aa + wCs ∆As + w A ∆Aas
where ∆Aa , ∆As and ∆Aas respectively are the incremental crack surface areas created in the
adhesive, the solids, and the interface between the solids and the adhesive. The onset and
progression of inelastic deformation and crack growth in a material is governed by the
condition of forces at the point of the crack and the material’s ability to sustain those forces.
The work of adhesion according to eq. (4) represents the thermodynamic bonding between the
adhesive and the carton board. High work of adhesion results if γ as is small, i.e. when the
energy of the molecules at the interface approaches the situation in the bulk of a material. This
requires strong bonding across the interface. Most of this bonding energy is due to
interactions across the interface such as Lifshitz-van der Waal and acid-base interactions (cf.
[6]). In addition to the thermodynamic bonding, diffusion and entanglement of polymer
molecules into the substrate may increase the adhesion as well as pure mechanical bonding
between the adhesive and the substrate. The latter contribution is particularly important for
fibrous materials such as carton board where mechanical bonding is accomplished by the
mechanical interlocking of the adhesive into irregularities in the carton-board surface and by
the hooking of fibres sticking out from the carton-board surface (see [9], [10]).
If the total bonding across the interface is large enough, the adhesive joint may fail by
cohesive fracture in the carton board or in the adhesive.
In polymer-based adhesives, such as hot-melt adhesives, the cohesive strength depends on the
molecular structure of the polymer (see [21]). Micromechanically, cohesive fracture occurs by
cavitation, fibrillation and crazing (see [22]). These processes involve a large portion of
inelastic deformation. Carton board is known to have inelastic deformation when subjected to
17
loading in the thickness direction (cf. [23]). This means that cohesive fracture involves energy
dissipation not only due to crack growth, but also due to inelastic deformation. Therefore, the
thermodynamic work of cohesion (i.e. wCa or wCs ) is of less importance when predicting the
energy needed for cohesive fracture of the joint.
Cohesive fracture in multi-layer carton board either takes place deep in the board structure
between the plies of the carton board (see [24]) or in the outermost ply adjacent to the
adhesive and close to the surface of the carton board. The former type of fracture is often
referred to as delamination, while the latter is referred to as tearing of the carton board. Note
that the difference between interfacial fracture and cohesive fracture in the carton board (by
tearing) is not distinct.
Interfacial fracture with partial tearing is revealed by the observation that parts of the gluestring area after fracture is covered by fibres. Complete tearing is when 100% of the glue
string is covered with fibres, and pure interfacial fracture is when no fibres at all stick to the
glue string. The expression “tearing” is normally used when more than 50% of the glue-string
area is covered by fibres after a fracture.
The interlaminar strength between the plies of a multi-layer carton board depends on how the
board is made, e.g. how the paper webs are dewatered and dried as well as whether starch is
added to improve the bonding between the board plies. The tearing strength and the bonding
strength of fibres to the surface of the carton board depend on the pulp composition, pulp
quality, added chemicals and a number of various process parameters. Many methods to
measure the behaviour and strength of carton board in the thickness direction exist, including
Z-toughness by [25] and Arcan by [23].
All in all, it appears that brittle failure is possible only if the joint fails by pure interfacial
fracture. The fact that no fibres are loosened from the carton board would then indicate that
the mechanical contribution to the interfacial strength is small in such cases. Maximum
toughness (i.e. dissipated energy) requires cohesive fracture, either in the carton board (by
delamination or by tearing) or in the adhesive.
4. Results and discussion
The newly designed adhesive applicator and the Y-peel test method have been used in
combination to test adhesively bonded carton board.
18
4.1 Typical failure modes
During the testing of 310 different samples, it was observed that the force-elongation curves
could be grouped into four main types of failure mode, M1–M4, Figure 10.
Mode
Characteristics
Example
2.0
single peak
sudden breakage at peak load
-
short displacement before breakage
-
no or very little recorded descending dissipative
1.5
ForceinN/mm
M1
-
1.0
0.5
energy
0.0
0
1
2
3
Pathin mm
-
single or multiple peaks
-
sudden breakage
-
relatively short displacement before breakage
-
small part of descending dissipative energy
1,2
1,0
Force in N/mm
0,8
M2
0,6
0,4
0,2
0,0
0,0
0,2
0,4
0,6
0,8
Path in mm
2,0
-
single or multiple peaks
-
no breakage within target max displacement
-
Force in N/mm
(3 mm)
M3
1,5
last part of force-elongation curve shows
1,0
0,5
approximately constant force
0,0
0
1
2
3
2
3
Path in mm
2,0
single or multiple peaks
-
significant displacement before breakage
-
moderate slope after peak
Force in N/mm
M4
1,5
-
1,0
0,5
0,0
0
1
Path in mm
Figure 10. Characteristic force-elongation curves in Y-peel testing with four main failure
modes.
The modes in Figure 10 are categorised by the shape of the force-elongation curves. Mode
M1 specifically has one single peak with an abrupt failure after a very short displacement with
no or very little descending dissipative energy, a pop-up failure. Mode M2 still has a quick
failure at a relatively short displacement, sometimes with several peaks, but it shows a small
but measurable amount of dissipative descending energy. Mode M3, on the other hand, does
not break completely within the specified maximum 3 mm displacement during testing.
Several peaks are not uncommon, and it has a significant proportion of dissipative descending
energy. The force is more or less constant during the final part of the force-elongation curve.
Finally, mode M4 is similar to M3, with a significant proportion of dissipative descending
19
energy, but after one or several peaks, the force will gradually decrease during elongation
until complete failure.
The majority of the 310 specimens had a mode M3 failure (Figure 11a). Mode M2 was the
least frequent failure, and M1 and M4 were more or less equal.
Maximal force in the force-elongation curves showed no correlation with the failure modes
except for failure mode M1 (Figure 11b), which appeared to have a generally lower peak
force and a higher standard deviation.
Only failure modes M3 and M4 had a significant proportion of dissipative descending energy,
WDdesc (Figure 11c). Failure mode M1 had in practice no W Ddesc at all.
Amount of different failure
Dissipative Descending Energy
Fmax
200
150
100
50
0
(mJ)
40
(N)
Number of
specimen
250
20
0
M1
M2
M3
M4
M1
M2
Failure mode
M3
M4
Failure mode
Figure 11a. Number of tested Y- Figure 11b. Fmax vs. failure
peel specimens with different
mode of 310 tested specimens
modes of failure. A Henkel
with the Henkel adhesive.
adhesive was used.
60
50
40
30
20
10
0
M1
M2
M3
M4
Failure mode
Figure 11c. WDdesc (dissipative
descending energy) vs. failure
mode of 310 tested specimens
with the Henkel adhesive.
4.2 Characterisation of failure modes
The force and elongation curves were recorded during all the tests. In some tests, a digital
microscope equipped with a video camera (TIMM 400 C, SPI GmbH, Oppenheim, Germany)
was used to take pictures of the joint during testing. The picture sequences were automatically
synchronised with the force-elongation curve (Figure 12). It was observed that the major
crack was initiated through small cracks during the ascending part. The small cracks then
merged during further elongation into a propagating major crack in the descending dissipative
part, which finally led to failure.
20
Figure 12. Numbered photographs from the formation and propagation of a delamination
fracture with corresponding positions in the Y-peel force-elongation curve numbered.
Characterised as fracture mode M3 with Fmax =36.8 N, W Ddesc =45.4 mJ.
In fracture mode M3 (Figure 12), the failure starts with small microcracks in the ascending
part of the curve (pictures 2–4). The microcracks appear between the bottom ply and the first
middle ply of the upper carton board in pictures 2–4. In the descending part, a major crack has
developed (picture 5) at the lower carton board. This crack leads to final failure with fibres
and the complete top ply being torn off from the lower carton board (picture 6). This is the
carton board on which the hot melt was applied in the adhesive applicator.
In modes M1 and M2, which have an abrupt failure with very small crack resistance after
crack initiation, the microcrack starts in the same way as in mode M3, but then the microcrack
grows very rapidly to a major crack that causes the final failure with no visible fibre tear in
mode M1 and some visible fibre tear in mode M2.
Mode M4 has a smooth curve, and the failure starts with the formation of microcracks in the
ascending part. The microcracks then merge in the descending part of the curve to form the
major crack leading to a final failure, where fibres abundantly have been torn off from either
of the carton-board surfaces.
In this study, the major crack always appeared in the adhesive, in the board ply closest to the
adhesive or at the interface between the outer and middle board ply. The failure never
occurred in the middle board plies or at the interface between middle plies.
4.3 Y-peel characterisation and fibre tear
In a trial, the open time parameter in the adhesive applicator was changed over an interval to
investigate the fracture process and how the crack propagated [11]. The idea was to test the
traditional method with manual peeling and that subjective evaluation of fibre tear would
correlate with a certain force-elongation curve or failure mode. The test showed that fibre tear
21
increases simultaneously with descending dissipative energy (Figure 13), until the strength of
the adhesive joint exceeds the internal strength of the fibre structure in the outer ply of the
board, leading to 100% fibre tear.
80
Descending Dissipative Energy (mJ)
LGT250
CRY425
70
60
50
40
30
20
10
0
0
25
50
75
100
Percent of fibre tear samples (%)
Figure 13. Per cent of fibre-tear samples vs. descending dissipative energy. Dashed arrows
indicate maximum internal strength of the outer plies of LGT and CRY.
5. Discussion
5.1 Y-peel test method
A test method that objectively measures the mechanical properties in adhesively bonded
carton board is the Y-peel method. To get good repeatability of the results, an adhesive
applicator was developed for sample preparation of adhesive joints between two carton-board
sheets. This laboratory adhesive applicator makes it possible to simulate the converting steps
in a production machine and independently set the converting parameters: open time, press
time, pressure on the joint, amount of adhesive, temperature, application speed and the choice
of adhesive. After the sheets have been glued together, the laminate is cut in pieces that are
suitable for the Y-peel test equipment, mounted and then torn apart in the uniaxial tensile
tester, in which the force-elongation curve is recorded. Three parts of the force-elongation
curve are identified: an elastic part, an ascending dissipative part and a descending dissipative
part. These three parts are the basis for post-evaluation and characterisation of the forceelongation curve.
5.2 Y-peel characterisation
From the force-elongation graphs, it is possible to draw conclusions about when and how the
crack propagates in the specimen.
-
The maximum measured force gives information about when the major crack
and the softening behaviour start.
The dissipative descending part gives information about how the major crack
propagates and how the failure behaves.
22
-
The ascending dissipative part has so far not shown any correlation fracture
characteristics or crack propagation, probably because the microcracks start
in the same way in all tested samples in this study.
In this study, a correlation between dissipative descending energy and fibre tear has been
found. Therefore, the dissipative descending energy can be used as a practical assessment of
the mechanical strength of an adhesive joint, a value that will reflect the experience of the
general converter. The maximal force Fmax, on the other hand, will give information about
when a crack in the joint is initiated, but it shows no obvious correlation with fibre tear.
The Y-peel method in combination with the adhesive applicator is an objective method to
measure and characterise when fracture is initiated and how good or bad the joint is, which
correlates well with practical experience (e.g. fibre tear).
5.3 Y-peel failure
Following the discussion in the introduction, it appears that brittle failure (i.e. fracture without
inelastic energy dissipation) would be possible only if the joint fails by pure interfacial
fracture. The fact that no fibres are loosened from the carton board would indicate that the
mechanical contribution to the interfacial strength is small in such cases. To explore this
matter, consider the brittle case of mode M2 with multiple peaks as in Figure 14. The fracture
initiates as an interfacial pop-up failure reflected by the first two peaks of the force-elongation
curve. Consider the second peak of the curve. From the video capture, the pop-up crack
growth was estimated to be 0.3 mm, and the width of the specimen is 25 mm. The grey areas
in Figure 14 represent the elastic energy released during the pop-up crack growth. This is
estimated to ∆Wel = -3.2 mJ. Tests made with controlled displacement, as in the Y-peel test,
lead to ∆Ω = 0 during fracture. The kinetic energy can be neglected because quasi-static
conditions are restored after a small amount of crack growth. Combining eqs. (8) and (9) then
gives
∆Wel + ∆Wie + wCa ∆Aa + wCc ∆Ac + w A ∆Aas = 0
(10)
Figure 14. The release of elastic energy during the pop-up crack growth.
The inelastic part of the dissipated energy corresponds approximately to the light grey area in
Figure 14. This estimate gives ∆Wie ≈ 0.65 mJ. No crack growth is visible in the adhesive,
23
i.e. ∆Aa = 0 , and there is no visible crack growth taking place in the carton board during the
pop-up fracture, implying ∆Ac ≈ 0 . The surface created in the interface due to the pop-up
crack growth is ∆Aas = 0.3 ⋅ 25 = 7.5 mm 2 . Inserting this into the above equation gives
∆W el + ∆Wie + w A ∆Aas = 0
(11)
Solving w A , based on experimental data, w A = 340 N/m.
Introducing Zisman’s empirical surface energy γ c into eq. (4) gives w A = γ a + γ c . Assuming
that the value for the surface energy of paperboard is γ c ≈ 55 mN/m [26] and for the cold
consolidated hot-melt adhesive is γ a ≈ 30 mN/m [27] would result in that w A = 0.085 N/m.
There is apparently a huge discrepancy between the surface energy derived from mechanical
testing compared to the surface energy derived from thermodynamic calculations, even in
cases behaving macroscopically “brittle”. This is in line with the conclusion by [12], who
stated that enhanced wetting gives higher work of adhesion. Gent and Schultz simply
introduced a magnification factor to cope with the fact that the experimentally measured work
of adhesion was much higher than the calculated thermodynamic work.
5.4 Demands on a joint
The Y-peel test in combination with the developed adhesive applicator makes it possible to
design the performance of an adhesive joint in carton-board packages or develop the cartonboard surface to match the adhesive for specific applications.
If the demands on the adhesive joint are a small amount of dissipative energy and if the joint
should open easily with little damage to the carton-board surface, then a seal with a forceelongation curve as in fracture mode M2 should be developed. This requires a combination of
a relatively strong carton-board surface and a matching surface energy of the adhesive that
permits the fracture to propagate at the interface between the board and the adhesive.
On the other hand, if it is important that the adhesive joint is tamper-proof but that relatively
low force is needed to open it, then a seal with a force-elongation curve such as the one for
fracture mode M3 should be developed. This in turn requires a relatively weak and porous
board surface structure with a matching viscosity and surface energy of the adhesive during
application to favour good wetting and mechanical interlocking of the board surface. This will
force the crack to propagate into the board structure and damage the surface. Moderation of
the force to open the joint can, in this case, be achieved by, for instance, changing the amount
of adhesive applied or the temperature of the adhesive during application, shortening the
opening time, or chemically modifying the adhesive itself.
6. Conclusions
The Y-peel method opens up a possibility for the converters to speak in relevant theoretical
terms and define their demands on adhesive joints quantitatively. It will be possible to
optimise the parameters for the converting process in terms of the combination of adhesive
and carton board for specific package applications. It will also be possible to further develop
24
the adhesive and carton board to make them more compatible with the aim of obtaining an
adhesive joint that works well throughout the entire packaging chain.
Future interesting research could be a finite element model of the Y-peel method to gain a
better understanding of what happens and which properties have the biggest influence for the
mechanical behaviour of an adhesive joint.
7. Acknowledgements
This work of research would not have been started or completed without the support and trust
of my previous manager Johan Tryding at former AssiDomän Frövi. I really appreciate the
interest he put in my work.
This work was made possible through the financial support from former AssiDomän Frövi,
now Korsnäs Frövi, and Sveaskog. I am most grateful for this support. Thank you, Ola
Karlsson (Korsnäs Frövi) for always believing in me and my work and for your support.
Thank you also to Lars Ödberg (Sveaskog and KTH) for always having time for me and all
my silly questions.
I would like to thank my project group and supervisors, who were there to encourage me and
give me good advice and theoretical support as my work proceeded. To my supervisor Gunnar
Engström (KAU) and vice supervisor Nils Hallbäck (KAU), with whom I co-wrote two
papers, our workdays, which often included weekends, have been full of fun, encouragement
and laughter. Vice supervisor Magnus Lestelius (KAU) has co-written two papers with me
and is one of the persons who taught me the fundamentals of research. Johan Tryding (former
AssiDomän Frövi, now TetraPak), also a co-writer of two papers, has been a great source of
inspiration.
I also want to thank all the people who were involved in the technical development of the
method and all the testing required for this research: Thomas Neumann, who drafted the
design of the adhesive applicator; Johan Granat, who participated in the design and
development of the adhesive applicator – it was always nice to talk with you; the support from
Åke Eriksson when brainstorming new test methods, addressing computer questions and
problems, and thinking about how the developed applicator could be further improved; Stefan
Andreasson, who supported further development of the Y-peel method; Inga-Lill Jansson,
who endlessly tested samples in the laboratory; and Roogher Johansson and Jonas Jarhäll for
supplying adhesive and providing encouragement. I also had a great deal of help from Robert
Junghans, Jakub Lewandowski, Therese Björklind and Anna Jansson with the preceding
theoretical and practical work in the form of diploma work, trainee work and master thesis
respectively, not to mention all my colleagues at Korsnäs Frövi who encouraged and
supported me.
Torbjörn Widmark has helped me with the writing of the thesis, for which I am truly grateful.
Thank you for your constant interest. Without your help, I do not know whether I would have
finished this thesis.
My special gratitude goes to everyone in my family. My wonderful wife, Sofia, who was
always there for me, supported and encouraged me when it was tough. My son, Evald, who
never understood why Dad had to work so much with the computer.
25
8. Reference
1
Å. Andersson, Nästan hela förpackningshistorien, Nord Emballage 10 pp 25-55.
(2004)
2
M. Löfgren, The leader of the pack, PhD thesis Karlstad University (2006)
3
D.Judd, B.Aalders and T.Melis, The silent salesman, Continental press, Singapore
(1989)
4
J. Tryding, M. Lestelius, C. Korin and J. Lewandowski, Interpretation of Y-peel
testing of adhesive sealed carton board, in: Verarbeitungsmaschinen und
Verpackungstechnik Dresden 2003 pp. 277-294 (2003).
5
Q. S. Xia, Mechanics of Deformation in Paperboard, PhD thesis, Department of
Mechanical Engineeering, Massachusetts Institute of Technology, Cambridge, MA
(2002)
6
J. C. Berg, The importance of acid-base interactions in wetting, coating, adhesion and
related phenomena, Nordic pulp and paper research journal, 8 (1): 75-85 (1993)
7
E.F. Eastman and L.Fullhart, Polyolefin and Ethylene Copolymer-based hot melt,
Handbook of adhesives 3rd edition 1990 I.Skeist ed
8
R.Joukio and S.Mansikkamäki, Papermaking Science and Technology, Volume 12,
page 239, Papet Oy, Helsinki (1998)
9
J. J. Bikerman, Adhesion to fibrous materials, Tappi Journal 44 (8): 568-570 (1961)
10
R. Pelton, W. Chen, H. Li and M. R. Engel, The peeling behaviour of pressure
sensitive adhesives from uncoated papers, Journal of Adhesion 77: 285-308 (2001).
11
C. Korin, J. Tryding. M. Lestelius, N. Hallbäck, Y-peel characterization of adhesivelybonded carton board-An objective method, Journal of Adhesion Science and
Technology 21 (2): 197-210 (2007)
12
A. N. Gent and J. Schultz, Effect of wetting liquids on the strength of adhesion of
viscoelastic materials, Journal of Adhesion 3, pp 281-294 (1972)
13
M. Edin, L. Ödberg and J. Sterte, Hot melt adhesion of liner sized with alkylketene
dimer, Nordic Pulp Paper Res. J., 17, 395 - 400 (2002)
14
A. J. Kinloch, C. C. Lau and J. G. Williams, The peeling of flexible laminates, Intl J.
Fracture, 66, 45–70 (1994).
15
R. P. Wool, Polymer Interface Structure and Strength, Hanser/Gardner Publication,
Cincinnati, OH (1995).
16
Q.S. Xia, M.C. Boyce and D.M. Parks, A constitutive model for the anisotropic
elastic-plastic deformation of paper and paperboard, Intl J. Solids Structures, 39: 4053
–4071 (2002).
26
17
C. Korin, N. Hallbäck and R. Junghans, Failure modes of adhesive joints in carton
board, Submitted for publication (2007)
18
M. E. Schrader, Effect of adsorbed vapour on liquid-solid adhesion, Contact Angle,
Wettability and Adhesion 3, pp 67-91 (2003)
19
M. K. Bernett and W. A. Zisman, Relation of wettability by aqueous solutions to the
surface constitution of low energy solids, Journal of physical chemistry 63 (8): 12411246, (1959)
20
A. A. Griffith, The phenomena of rupture and flow in solids, Philosophical
transactions of royal society, A221, 163-173 (1921)
21
I. Skeist and J. Miron, Introduction to adhesives, Handbook of adhesives 3. ed., I.
Skeist (ed), Van Nostrand Reinhold, cop., New York, 1990.
22
H. J. Sue, Craze-like damage in a core-shell rubber-modified epoxy system, Journal of
Material Science 27, pp 3098-3107 (1992).
23
N. Stenberg, C. Fellers and S. Östlund, Measuring the stress-strain properties of
paperboard in the thickness direction, Journal of pulp and paper science 27 (6): 213221 (2001).
24
Q S Xia, Mechanics of Deformation in Paperboard, PhD thesis, Department of
Mechanical Engineeering, Massachusetts Institute of Technology, Cambridge, MA
(2002)
25
A. Lundh and C. Fellers, The Z-toughness method for measuring the delamination
resistance of paper, Nordic Pulp and Paper Research Journal 16 (4): 298-305 (2001).
26
L. Wågberg and G.Annergren: Physico-chemical characterisation of papermaking
fibres, Trans. 11th Fundamental Research Symposium. Cambridge Vol 1 p1, Pira
International, Leatherhead, UK (1997)
27
M.F. Tse, Hot Melt Adhesive Model: Interfacila Adhesion and Polymer/Tackifier/Wax
interactions, J. Adhesion, 66, 61-88 (1998)
27
9. Notations
MD
The machine direction, the direction in which the board is
produced on the board machine
CD
The cross-machine direction, the direction in which the board is
produced on the board machine
ZD
The out-of-plane direction, i.e. the thickness direction
CTMP
Chemi-thermo-mechanical pulp
P
The load acting on the upper simply supported clamp
R
The reaction force at the two simply supported clamps at the lower
part of the set-up
α
The peel angle
Ft
The reaction forces in vertical direction
Fn
The reaction force in horizontal, normal to the board surface
M
The moment
u
The total elongation
ue
The elastic part of u
δ nie
The inelastic deformation of the adhesive joint; the inelastic
opening of the adhesive joint in the direction of Fn
Fn max
The maximum force in the normal direction
Welremain
The remaining elastic energy
W Dasc
The energy that is consumed during the ascending part of force
versus inelastic deformation
W Ddesc
The energy that is consumed during the descending part of force
versus inelastic deformation
W Dinitial
The initial energy caused by initial
wCa
The work of cohesion in the adhesive
28
γa
The surface tension of the adhesive
wCs
The work of cohesion in the carton board
γs
The surface tension of the carton board
γ as
The surface tension of the interface
θ
The contact angle
γc
The empirical critical surface tension
∆Ω
The energy put into the system by external forces
∆W
The change in strain energy
∆W f
The total surface energy consumption during crack growth
∆Wel
The elastic part of the strain energy
∆Wie
The inelastic part of the strain energy
∆Aa
The incremental crack surface areas created in the adhesive
∆As
The incremental crack surface areas created in the solids
∆Aas
The incremental crack surface areas created in the interface
between the solids and the adhesive
Fmax
The maximal force
29
Fracture Behaviour of
adhesive Joints in
carton board
A carton-board package is often sealed and closed with an adhesive. Package requirements
vary depending on how the package is to be used. A package that is only supposed to protect
the product during transport differs from one that is supposed to attract consumers and
facilitate the use of the product by consumers. Fracture of the adhesive joint may occur in
several different ways, e.g. cohesive fracture in the adhesive, interfacial fracture between
the adhesive and one of the carton-board surfaces, and cohesive fracture in the carton
board. The traditional way of testing the adhesive joint is to subjectively evaluate the fibre
tear after manually tearing the joint apart. The adhesives used in carton-board packages are
either hotmelt adhesives (adhesives that are applied in a molten state on the carton board)
or dispersion adhesives (adhesives that are applied as water-based dispersions).
The primary interest of this study has been to find an objective method that can characterise the adhesive joint, i.e. the strength and joint characteristics. The work has principally
concentrated on physical experiments where the Y-peel method is evaluated and further
developed, including construction of a laboratory adhesive applicator.
Adhesive joint failure is analysed and correlated to the force-elongation curve in order to
explore various mechanisms of adhesive-joint failure. The force versus elongation curves
are transformed into a force versus inelastic deformation curve for the adhesive joint. The
inelastic deformation of the adhesive joint is defined as the inelastic opening of the adhesive joint perpendicular to the carton-board surface. The dissipative descending energy has
been used to evaluate the adhesive joint. High descending dissipative energy resulted in
high resistance against final failure of the joint. This correlates very well with the manual
fibre-tear test. Characteristic force-elongation curves in Y-peel testing, i.e. the shape of the
curve, have been analysed and found to have four main failure modes.
Using both the newly designed adhesive applicator and the Y-peel test method revealed a
large discrepancy, about three orders of magnitude, between the theoretical thermodynamic
analysis and the experimental test result. The new objective method can be used to design the
interaction between the adhesive and the carton-board surface for a specific application. This
can be achieved by modifying the carton board, the adhesive or the process parameters.
Karlstad University Studies
ISSN 1403-8099
ISBN 978-91-7063-138-7